How can Ufactor be the same for low-e coated and uncoated glasses?

[Volkan Özcan]

Hello,

I'm learning to use the software. Thank you for creating it.

I'm confused about one of the results.

In glazing system library tab, I am creating two glazing systems that are single layers. One of them is "Planibel Green 8 mm" by AGC and other is "Sunenergy Green 8mm" by AGC. Sunenery is a low-e coated glass (https://www.agc-yourglass.com/gb/en/brands/sunergy). I'm not sure if they use planibel green to produce sunenery or if they use a different shade of green.

The part I'm confused is, Window 7.7 is calculating almost the same Ufactor for both of the glasses. (see attahed images.)

I thought it should have calculated different Uvalues, since one of them has a low-e coating. I tried every environmental conditions and thermal calculation standard and flipping the coated side but Uvalue does not change. 

I repeated the same calculation on AGC's performance calculator. AGC's results and Window's results are pretty much the same of the Planibel (uncoated) glass. However, while AGC decreases the Uvalue for the coated glass (as it should), Window keeps the same value. I attached pdfs from AGC's calculator.

So, is there a problem with Window's result or am I missing something?

Best wishes,

[Robin Mitchell]

I think it has to do with using the CEN boundary conditions, which use a fixed combined coefficient.

If you use the NFRC 100-2010 boundary conditions, which include radiative effects, the difference between the two is significant.

Robin

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Robin Mitchell

Building Technology and Urban Systems Division
Energy Technologies Area
Lawrence Berkeley National Laboratory

[Robin Mitchell]

Others that are more familiar with the CEN world can probably provide more insight. 

Robin 

[Paul Caicedo]

Hello,

Why the difference in the Ufactor is high when flipping the film. It should be the same

Regards,

Paul

[Dragan Vidanovic]

Paul,

Because if the IR radiation. Try creating BC that do not have IR and see if results are going to be identical.

Best,

Simon

[D. Charlie Curcija]

To complement Simon's response, low-e coating has a much bigger effect in the gap space vs facing outdoors. Thermal radiation is about half of heat transfer in the gap (other half being natural convection), while it is only a small fraction on the outdoor surface, with much bigger portion being forced convection (wind induced). The difference in U-Factor should be large.

Charlie

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[Robin Mitchell]

Paul

It is thermally very different to have the low-E coating facing the gap vs having it on the exterior of the glazing system. 

Radiation heat transfer is very sensitive to the temperature difference of the surfaces facing each other. 

In the case of a double pane glazing system, the two panes of glass are at very different temperatures because the air space is providing good insulation, resulting in a large radiant exchange between the two layers across the gap that can be avoided by a low-E coating on either of the gap-facing surfaces.  

When the low-E coating is on surface 1 or 4, which are the outer / inner most surfaces of a double pane unit, the temperature difference that is relevant for radiation is the glass surface temperature vs the surrounding environmental temperature. This is typically a smaller temperature difference than the temperature difference across the gap in the glazing system. 

Therefore the performance is not likely to be the same when putting the coating on an gap-facing glass surface vs an exterior glass surface. In particular, when you put the coating on the outside, the standard environmental condition assumes a high wind speed which further reduces the radiant portion because the temperature difference between the glass and the environment is smaller. 

We are not aware of any commercially available low-E products where the coating faces the exterior, although there are many products with a surface 4 low-E facing the room that is in addition to a low-E coating facing the gap. 

So the fact that WINDOW is showing a higher U-factor for the case where the low-E coating is facing the exterior is correct. 

Robin Mitchell and Howdy Goudey

Building Technology and Urban Systems Division

Energy Technologies Area
Lawrence Berkeley National Laboratory

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